A tense meeting of Democrats on the Senate Environment and Public Works Committee saw Chairwoman Barbara Boxer and her chief aide squaring off against senators who support a bipartisan agreement to overhaul the nation’s toxic chemical laws.

The agreement, championed by the late Sen. Frank R. Lautenberg in the weeks before his June 3 death, has come under fire from Boxer for being weaker than some state laws, particularly in her home state of California. Multiple sources said Boxer began the closed-door meeting by announcing she would convene a markup with Lautenberg’s original Safe Chemicals Act, which didn’t have bipartisan support.

She encountered near-immediate resistance, however, from some colleagues who bristled after she accused them of not understanding the policy prescriptions contained in the compromise bill authored by Lautenberg and ranking member David Vitter of Louisiana.

Sources said the meeting in Boxer’s personal office became tense when Boxer repeatedly suggested that Democratic co-sponsors of the compromise bill would not have supported it if they truly understood what was in the bill.

At one point, Sen. Tom Udall, D-N.M. — who chairs the Superfund, Toxics and Environmental Health subcommittee — cut Boxer off, multiple sources said. Udall told Boxer he understood what he was supporting when he agreed to be a named co-sponsor, had read the bill and would continue to support the legislation.

Udall, like other members in the room, suggested that the compromise bill has the best chance of successfully updating the nearly 40-year old Toxic Substances Control Act.

EPW Staff Director Bettina Poirier then directly challenged Udall and his characterization of the legislation, sources said. That irked Sen. Sheldon Whitehouse, D-R.I., who backed Udall, expressing his displeasure with the tone Poirier had taken with Udall, as well as with Boxer’s general approach to the intraparty debate on the bill. Whitehouse is not a co-sponsor of the Lautenberg-Vitter legislation.

Spokesmen for Udall and Whitehouse declined to comment about the meeting.

Boxer convened the meeting after CQ Roll Call reported Tuesday that she had begun an aggressive lobbying campaign to dismantle the bill and that she had questioned Lautenberg’s capacity to agree to the compromise, given the illness that took his life two weeks after he announced the deal with Vitter.

After Thursday’s meeting, however, aides who support the Lautenberg-Vitter legislation felt cautiously optimistic that it might be used as the base bill for a future markup on toxic chemical legislation. However, they still harbor some doubts on how a markup might actually go down. According to sources, there was consensus among the rank-and-file EPW members that the bipartisan bill, though in need of amendment, was the best framework moving forward because it had conservative backing.

There remains some dispute over how much the committee markup might rely on the existing compromise bill. While some staffers took away from the meeting a commitment from Boxer that she would move forward with the bill as a base, others said the commitment was to use the outline of the bill but not the language.

Boxer has said she will hold a hearing in July on the issue.

The California Democrat is not the only obstacle to the measure. Environmental groups have raised concerns about the Lautenberg-Vitter bill, saying it doesn’t go far enough in instituting new protections.

In a statement issued in May, Environmental Working Group President Ken Cook said: “Gone from the bill are all the vital provisions that gave the public hope that Congress would stand up to chemical companies and toughen federal safeguards specifically to protect infants, children and expectant mothers from the toxic industrial chemicals all around us. From its findings and policy goals straight through to its regulatory provisions, this bill falls far short of meeting the pressing need to protect public health and the environment.”